Bits of this type are known and disclosed, for example, in U.S. Pat. Nos. 4,098,362 and 4,244,432. The cutting elements of the bits disclosed in these patents are preformed cutters in the form of cylinders that are secured to the bit body either by mounting the elements in recesses in the body or by brazing or soldering each element to a pin which is fitted into a recess in the bit body. During drilling impacts exerted to the cutting elements are severe and in order to accomplish that undue stresses in the elements are avoided the frontal surface of each element is generally oriented at a negative top rake angle between zero and twenty degrees.
The abrasive particles of the front layers of the cutting elements are usually synthetic diamonds or cubic boron nitride particles that are bonded together to a compact polycrystalline mass. The front layer of each cutting element maybe backed by a cemented tungsten carbide substratum to take the thrust imposed on the front layer during drilling. Preformed cutting elements of this type are disclosed in U.S. Pat. No. 4,194,790 and in European Pat. No. 0029187 and they are often indicated as composite compact cutters, or--in case the abrasive particles are diamonds--as polycrystalline diamond compacts (PDC).
During drilling, the cutting elements of a bit run along concentric tracks that overlap each other so that the concentric grooves carried by the various cutting elements in the borehole bottom cause a uniform deepening of the borehole. The elements thereby provide aggressive cutting action to carve the grooves in the bottom and, during drilling, the temperature at the cutting edge of the elements may raise several hundreds degrees Celsius above the formation temperature. The temperature at the cutting edges of this temperature should, in the nowadays applied compact cutters, not exceed 750.degree. C. Above this temperature the bonds between the abrasive particles are weakened to an undue extend so that the particles can easily be pulled out from the matrix, thereby causing an excessive increase in bit wear.
Detailed inspection of field worn drill bits revealed that the abrasive front layers of the cutting elements show wear at the cutting edge only. This wear mechanism has an almost steady state nature since in general the front layers appear to be worn in such a manner that the cutting edge thereof attacks the rock at a negative rake angle, generally indicated as the wear-angle, of between 10.degree. and 15.degree. relative to the borehole bottom. The substrate layers backing the front layers of the elements appear to be worn off substantially parallel to the borehole bottom; the flat surface thus formed at the underside of the element is generally indicated as the wear-flat.
As known to those skilled in the art of drilling, the steady state of the rake angle at the cutting edge is a consequence of shaped body of crushed rock between the toe of the cutting element, the virgin formation and the chip being scraped therefrom. This body of crushed or even plastic rock, called the build-up edge, is of major importance to the drilling performance of the cutting element. This can be illustrated by the fact that under similar drilling conditions (i.e., identical speed of rotation and penetration rate) the drill cuttings in the return mud flow of a worn drill bit are upset to a greater extend than the drill cuttings of fresh bit. The increased upsetting of the drill cuttings is a consequence of the presence of the build-up edge at the toe of a worn cutting element. The contact surfaces between the build-up edge, the chip and the virgin formation, at which surfaces rock to rock contact occurs, form areas of extremely high friction at which a large amount of frictional heat is generated during drilling.
Moreover it appeared that in field worn bits that had been driven by a rotating drill spring at a speed of rotation of typically one hundred revolutions per minute, the front layers of the elements were worn away at the toe thereof in such manner that the cutting edge is located at the interface between the front layer and the substratum. The cutting elements of field worn bits that had been driven by a down-hole turbine at a relatively high speed of rotation of typically about eight hundred revolutions per minute appeared to be worn away in such a manner that the cutting edge thereof is located at about 0.3 mm behind the frontal surface of the front layer.
The cutting elements of these field worn bits were provided as usual, with an abrasive front layer having a thickness of about 0.6 mm. Hence the cutting edge of a cutting element of such a field worn turbine driven bit is located about halfway between the frontal surface of the front layer and the interface between the front layer and substratum, which implies that, during turbine drilling, the section of the lower surface of the front layer behind the cutting edge forms part of the wear-flat. As friction between the abrasive particles of the front layer and the rock formation is high in comparison to friction between the lower surface of the substratum and the rock formation, an excessive amount of frictional heat is generated during turbine drilling at the section of lower surface of the front layer behind the cutting edge.